Patients suffering from different forms of breathing disorders can be subject to several types of treatments depending on the illness or disorder present. One form of treatment is to use a mechanical ventilator as it represents a non-invasive technique for the treatment of certain breathing disorders such as ventilatory failure, hypoventilation, periodic breathing during sleep and wakefulness, and in sleep apnea that occurs exclusively during sleep.
Ventilatory failure comprises all forms of insufficient ventilation with respect to a metabolic need which may occur during wakefulness or periods of sleep. Hypoventilation and periodic breathing, in its most frequently occurring form referred to as Cheyne-Stokes ventilation, may occur periodically or constantly during wakefulness or sleep. Conditions associated with hypoventilation, in particular nocturnal hypoventilation, may comprise, for example, central nervous system disorders such as stroke, muscular dystrophies, certain congenital conditions, advanced chronic obstructive pulmonary disease (COPD), etc. Cheyne-Stokes ventilation or various forms of central apnea are commonly associated with cardiac and circulatory disorders, in particular cardiac failure.
Ventilatory failure is a potentially life threatening condition. The general comorbidity in patients with failing ventilation is considerable. The condition is highly disabling in terms of reduced physical capacity, cognitive dysfunction in severe cases and poor quality of life. Patients with ventilatory failure therefore experience significant daytime symptoms but in addition, the majority of these cases experience a general worsening of their condition during state changes such as sleep. The phenomenon of disordered breathing during sleep, whether occurring as a consequence of ventilatory failure or as a component of sleep apnea in accordance with the description above causes sleep fragmentation. Daytime complications include sleepiness and cognitive dysfunction. Severe sleep disordered breathing occurring in other comorbid conditions like obesity, neuromuscular disease, post polio myelitis states, scoliosis or heart failure may be associated with considerable worsening of hypoventilation and a compromised blood gas balance. Sleep apnea has been associated with cardiovascular complications such as, for example, coronary heart disease, myocardial infarction, stroke, arterial hypertension, thrombosis, and cardiac arrhythmia. It is therefore of both immediate and long-term interest to reduce the exposure to sleep disordered breathing.
In ventilation treatment it is crucial that the inspiratory and expiratory phases of the applied system synchronize with the patient's own ventilation efforts. A system that does not comply with patient effort creates an air trapping situation and increases discomfort as well as a considerably increased anxiety for the patient. Several different systems have been developed to make mechanical ventilators operate in accordance with patient rhythms. The key component in such systems is the ability to sense the patient's rhythm or breathing phases. The early systems used pressure sensing devices to detect the pressure drop when a patient tried to inhale.
Flow systems, which succeeded the pressure systems, were based on a flow sensing mechanism which sensed the flow of gas entering and leaving the patient. There have been a number of methods of measuring the instantaneous and average gas flow described in literature to calculate the inspiratory and expiratory triggering points.